The prior art discloses numerous techniques for analyzing a subject's pulmonary function. In one such technique, the subject takes a single breath of an inert aerosol, and the aerosol concentration is determined upon inspiration and expiration. The resulting difference is attributed to aerosol deposition in the subject's airways. Differing levels of aerosol deposition as between subjects may be used, for example, as an indication of increased airway obstruction, which is known to cause enhanced aerosol deposition. This technique, however, is relatively insensitive to small variations in pulmonary function, and hence has limited applications. In addition, it appears that the accuracy of the data derived from the single breath method may be rendered inaccurate due to effects of aerosol dilution, as described hereinafter. The prior art single aerosol deposition technique is disclosed, for example, in Distribution of Aerosol Particles in Exhaled Air, Muir, Journal of Applied Physiology, Vol. 23, No. 2, 1967, The Effect of Airways Obstruction on the Single Breath Aerosol Curve, Muir, as appearing in Airway Dynamics by Bouhys, 1970 Edition, pp. 319-325, and Aerosol Transport in the Human Lung From Analysis of Single Breaths, Taulbee et al., Journal of Applied Physiology, Vol. 44, No. 5, pp. 803-812, 1978.
Evaluation of pulmonary function by analysis of aerosol deposition data has also been carried out for single breaths by scanning the lungs after inhalation of a radioactive aerosol. Such studies are described, for example, in Early Detection of Chronic Obstructive Pulmonary Disease Using Radionuclide Lung-Imaging Procedures, Taplin, et al., Chest, Vol. 71, pp. 567-575, 1977, and Imaging Sites of Airway Obstruction and Measuring the Functional Responses to Bronchodilator Treatment in Asthma, Chopra, et al., Thorax, Vol. 34, pp. 493-500, 1979. The obvious disadvantage of this technique is that due to effects of radiation exposure, it cannot be repeated to follow serial changes after therapeutic intervention.
The prior art also recognizes that accumulated secretions in the airways may result in a two phase gas-liquid flow. Resistance to Two-Phase Gas-Liquid Flow in Airways, Clarke et al., Journal of Applied Physiology, Vol. 29, No. 4, 1970. However, the effect of this phenomenon on aerosol deposition has not heretofore been studied.